Vassilios G. Agelidis

An Input Current Feedback Method to Mitigate the DC-Side Low-Frequency Ripple Current in a Single-Phase Boost Inverter

A boost DC/AC converter is popular in AC line-integrated energy storage systems (ESSs) based on low-voltage DC sources such as battery, fuel cell, or supercapacitor. The direct DC/AC power conversion in the boost inverter introduces a second-order harmonic ripple current at the DC side of the boost converter, which leads to internal heating of the energy storage devices and degradation of their lifetime. In this paper, a novel current feedback method is proposed to mitigate the second-order harmonic current component at the DC side. The proposed method is able to significantly reduce the second-order harmonic component in the DC-side current of the boost inverter without increasing other harmonic components. Performance of the proposed method is validated on a single-phase grid-connected DC/AC boost inverter-based battery ESS experimental prototype.

Supercapacitor Sizing Method for Energy-Controlled Filter-Based Hybrid Energy Storage Systems

Filter-based battery-supercapacitor hybrid energy storage systems (HESSs) are popular as a way of extending battery lifetime by diverging the high-frequency power variations to the supercapacitor. However, when a traditional supercapacitor voltage controller (SCVC) is employed in the filter-based HESS, precise sizing of the supercapacitor as well as finding filter parameters for the power allocation are challenging due to nonlinearities. These problems can be circumvented by using a supercapacitor energy controller (SCEC) proposed in this paper. The paper presents a method for selection of the SCEC and filter parameters as well as precise sizing of the supercapacitor for a given application. The proposed method is experimentally verified on a single-phase grid-connected HESS used to smooth the power delivered to the grid at the point of common coupling. It is also shown that the size of the supercapacitor when using the SCEC is significantly lower than the one estimated for the traditional SCVC.

A Rule-Based Controller to Mitigate DC-Side Second-Order Harmonic Current in a Single-Phase Boost Inverter

Single conversion stage DC/AC boost inverters are an attractive solution when integrating energy storage devices, such as a battery, fuel cell, or supercapacitor to a single-phase AC grid. However, a second-order harmonic current ripple appears at the DC side of the inverter increasing the internal heat and losses in the energy storage device and degrading its lifetime. In this paper, a rule-based controller is proposed to reduce such harmonic current ripple component. A key feature and advantage of the proposed controller is its ability to reduce the ripple current amplitude in all four inverter output power operating quadrants without being affected by the capacitor tolerances and the internal resistance of the inductors. Presented experimental results validate the performance of the proposed controller on a single-phase grid-connected DC/AC boost inverter-based battery energy storage system.

A Fixed-Frequency Sliding Mode Controller for a Boost-Inverter-Based Battery-Supercapacitor Hybrid Energy Storage System

The boost-inverter-based battery-supercapacitor hybrid energy storage systems (HESSs) are a popular choice for the battery lifetime extension and system power enhancement. Various sliding mode (SM) controllers have been used to control the boost inverter topology in the literature. However, the traditional SM controllers for the boost inverter topology operate with a high and variable switching frequency which increases the power losses and system components design complexity. This can be alleviated by a pulse width modulation (PWM)-based fixed-frequency SM controller proposed in this paper. The SM controller is implemented using variable amplitude PWM carrier signals generated using the output capacitor voltage and inductor current measurements, thus eliminating the requirement of the output capacitor currents measurement. The battery-connected inductor reference currents for the SM controller are generated by a supercapacitor energy controller which is responsible for the HESS power allocation. First, the theoretical aspects of the SM controller, the operation and parameter selection of the supercapacitor energy controller, and the supercapacitor sizing for the HESS are discussed in the paper. Then, the proposed control system is experimentally verified, and it is shown that the HESS is able to satisfy the HESS output power requirements, while allocating the ripple current and the fast power fluctuations to the supercapacitor while maintaining operation of the supercapacitor within predefined voltage limits. The main advantage of the proposed SM controller, as compared with the traditional double-loop control method, is in eliminating possible DC current injection into the grid when the equivalent series resistance values of the boost inductors become unequal due to the tolerances and temperature variations.

Integrated Reconfigurable Configuration for Battery/Ultracapacitor Hybrid Energy Storage Systems

In conventional parallel hybrid energy storage systems (HESSs), there are usually two cascaded converters active when transferring energy between its energy storage systems (ESSs) that result in the overall efficiency drop. Moreover, the dc-link capacitor has to be oversized to limit the voltage ripple during simultaneous energy exchange between the ESSs and load feeding. The integrated reconfigurable configuration for HESSs proposed in this paper allows direct energy transfer between the ESSs by using only a single converter and bypassing the dc link. This reduces the stress on the dc link, reduces the dc-link capacitor size, and improves the efficiency. The proposed configuration can be reconfigured to operate in different operating modes: “feeding a load” mode, “regenerative” mode, “energy exchange” mode, “energy exchange and feeding a load” mode, “balancing” mode, “charging” mode, and “backup” mode. To control the proposed HESS, a battery peak power limiting method is used to allocate the power components between the ESSs. The operating principle of the proposed configuration and its different modes are explained and experimentally verified.

Control Strategies for Microgrids With Distributed Energy Storage Systems: An Overview

This paper presents an overview of the state of the art control strategies specifically designed to coordinate distributed energy storage (ES) systems in microgrids. Power networks are undergoing a transition from the traditional model of centralised generation towards a smart decentralised network of renewable sources and ES systems, organised into autonomous microgrids. ES systems can provide a range of services, particularly when distributed throughout the power network. The introduction of distributed ES represents a fundamental change for power networks, increasing the network control problem dimensionality and adding long time-scale dynamics associated with the storage systems’ state of charge levels. Managing microgrids with many small distributed ES systems requires new scalable control strategies that are robust to power network and communication network disturbances. This paper reviews the range of services distributed ES systems can provide, and the control challenges they introduce. The focus of this paper is a presentation of the latest decentralised, centralised and distributed multi-agent control strategies designed to coordinate distributed microgrid ES systems. Finally, multi-agent control with agents satisfying Wooldridge’s definition of intelligence is proposed as a promising direction for future research.

A single phase grid integration scheme for battery-supercapacitor AC line hybrid storage system

A transformerless direct single phase grid-connected battery-supercapacitor hybrid energy storage system based on a modified boost inverter configuration and its associated control strategy are proposed in this paper. The proposed control strategy comprises a double loop output voltage controller, active and reactive power droop controller, battery state of charge (SOC) management system based on the extended Kalman filter (EKF) and supercapacitor voltage controller. Simulation results confirm that the proposed system is able to deliver or absorb the required active power and reactive power to or from the grid while allocating the ripple current to the supercapacitor. Furthermore, the system maintains the supercapacitor voltage at a pre-defined value while operating the battery inside a safe operating SOC range.

Delta-Connected Cascaded H-Bridge Multilevel Converters for Large-Scale Photovoltaic Grid Integration

The cascaded H-bridge (CHB) converter is becoming a promising candidate for use in next generation large-scale photovoltaic (PV) power plants. However, solar power generation in the three converter phase-legs can be significantly unbalanced, especially in a large geographically-dispersed plant. The power imbalance between the three phases defines a limit for the injection of balanced three-phase currents to the grid. This paper quantifies the performance of, and experimentally confirms, the recently proposed delta-connected CHB converter for PV applications as an alternative configuration for large-scale PV power plants. The required voltage and current overrating for the converter is analytically developed and compared against the star-connected counterpart. It is shown that the delta-connected CHB converter extends the balancing capabilities of the star-connected CHB and can accommodate most imbalance cases with relatively small overrating. Experimental results from a laboratory prototype are provided to validate the operation of the delta-connected CHB converter under various power imbalance cases.

Multi-Agent Sliding Mode Control for State of Charge Balancing Between Battery Energy Storage Systems Distributed in a DC Microgrid

This paper proposes the novel use of multi-agent sliding mode control for state of charge balancing between distributed dc microgrid battery energy storage systems. Unlike existing control strategies based on linear multi-agent consensus protocols, the proposed nonlinear state of charge balancing strategy: 1) ensures the battery energy storage systems are either all charging or all discharging, thus eliminating circulating currents, increasing efficiency, and reducing battery lifetime degradation; 2) achieves faster state of charge balancing; 3) avoids overloading the battery energy storage systems during periods of high load; and 4) provides plug and play capability. The proposed control strategy can be readily integrated with existing multi-agent controllers for secondary voltage regulation and accurate current sharing. The performance of the proposed control strategy was verified with an RTDS Technologies real-time digital simulator, using switching converter models and nonlinear lead-acid battery models.

Model Predictive Control for Distributed Microgrid Battery Energy Storage Systems

This brief proposes a new convex model predictive control (MPC) strategy for dynamic optimal power flow between battery energy storage (ES) systems distributed in an ac microgrid. The proposed control strategy uses a new problem formulation, based on a linear <inline-formula> <tex-math notation=LaTeX>